1. Field of the Invention
The present invention relates to a controller for a motor adapted to carry out field weakening control of a permanent magnet field type rotary motor by changing a phase difference between two rotors disposed around a rotating shaft.
2. Description of the Related Art
Hitherto, there has been known a permanent magnet field type rotary motor which is equipped with a first rotor and a second rotor concentrically provided around a rotating shaft thereof and which is adapted to conduct the field weakening control by changing the phase difference between the first rotor and the second rotor according to a rotational velocity (refer to, for example, Japanese Unexamined Patent Application Publication No. 2002-204541).
In such a conventional motor, the first rotor and the second rotor are connected through the intermediary of a member that is displaced in the radial direction when subjected to a centrifugal force. The motor is configured such that, when the motor is in a halting state, the magnetic poles of the permanent magnets disposed in the first rotor and the magnetic poles of the permanent magnets disposed in the second rotor are oriented in the same direction, providing a maximum magnetic flux of the field, while the phase difference between the first rotor and the second rotor increases due to a centrifugal force as the rotational velocity of the motor increases, thus reducing the magnetic fluxes of the field.
FIG. 12 shows a range in which the fields of the motor need to be weakened. In the figure, the axis of ordinates indicates output torque Tr and the axis of abscissas indicates the number of revolutions N. The character “u” in the figure denotes an orthogonal line of the motor. The line u is formed by connecting points at which a phase voltage of the motor becomes equal to a supply voltage, depending on a combination of the number of revolutions and an output torque when the motor is actuated without carrying out the field weakening control. The character X in the figure denotes a range in which the field is not required to be weakened, while Y denotes a range in which the field is required to be weakened.
As shown in FIG. 12, the range Y in which the field need to be weakened is determined by the number of revolutions N and the output torque Tr of the motor. Hence, the conventional field weakening control, which depends merely on the number of revolutions, tends to inconveniently result in an excessive or insufficient control amount for weakening the field.
Basically, the field weakening control is intended to reduce a back electromotive force produced in an armature by the revolution of the motor so as to restrain a voltage between terminals of the armature from becoming higher than a supply voltage, thereby allowing the motor to be used in a higher revolution range. When changing the phase difference between the first rotor and the second rotor by the number of revolutions of the motor or a centrifugal force, only the number of revolutions is the parameter for changing the weakening of the field. This inconveniently prevents flexible changes of the controllable range of output torque or the number of revolutions of the motor.
Furthermore, in a motor that operates also as a generator, the operating efficiency is generally improved by using different field control amounts for a driving mode (positive output torque) and a power generating mode (negative output torque), respectively, when the number of revolutions remains the same. However, when changing the phase difference between the first rotor and the second rotor by the number of revolutions or a centrifugal force, it is disadvantageous that the field control amount cannot be changed between the driving mode and the power generating mode.